\documentclass{standalone}
\usepackage{tikz}
\usepackage{ctex,siunitx}
\setCJKmainfont{Noto Serif CJK SC}
\usepackage{tkz-euclide}
\usepackage{amsmath}
\usepackage{wasysym}
\usetikzlibrary{patterns, calc}
\usetikzlibrary {decorations.pathmorphing, decorations.pathreplacing, decorations.shapes,}
\begin{document}
\small
\begin{tikzpicture}[>=latex,xscale=0.5]
  % \useasboundingbox(0.9,0)rectangle(5.1,5);
  \draw [<->](0,1.2)node[right]{$i_R$}--(0,0)node[below left]{$O$}--(14,0)node[right]{$t$};
  \foreach \y in {1,2,3,4} { \draw [dashed] (pi*\y, 0)--(pi*\y, 1.2); }
  \draw [ultra thick] plot[domain=0:pi, samples=1000] function{0.8*sin(x)};
  \draw [ultra thick] plot[domain=2*pi:3*pi, samples=1000] function{0.8*sin(x)};
\end{tikzpicture}
\end{document}